Vane pumps, in particular, are used very frequently for supplying auxiliary power steering mechanisms with pressure oil. Narrowing the pressure channel at an output flow point is a known way of controlling the stream of oil being delivered for the auxiliary power steering mechanism. This method causes a higher velocity of flow, and consequently a reduction of the static pressure on the booster piston or consumer end of the flow-regulating piston. Thus, the differential pressure between the flow regulating-piston end surfaces become greater and the flow-regulating piston is caused to shift responsive to the rate of pump speed, which is dependent on engine speed and thus corresponds to vehicle speed. Consequently, the flow admission port into the bypass or return channel for oil flow from pump outlet to inlet, or suction side, is opened to a corresponding extent. Accordingly, more hydraulic fluid is recirculated in the pump and the flow of oil to the consumer decreases. A dropping or falling characteristic curve of the vane pump occurs, i.e., the pump discharge is not increased with the work done by the vane pump after a certain cutoff point is reached, but is even reduced, if desired.
Certain characteristic curve variants are desired for various reasons. Thus, or example, a falling characteristic curve provides better steering operation in an auxiliary power steering mechanism at high speeds. Furthermore, the booster pressure in the steering system decreases when the characteristic curve of the flow of oil is falling if the speed is increasing. The result is a smaller consumption of power by the pump, and that effects lower temperatures.
For energy saving reasons and to avoid high oil temperatures, it is desirable to have small oil volume flowing through the booster system, especially when driving on a high-speed straight stretch of road. On the other hand, a high flow of oil should be available for a steering operation when operating pressure is high.
A known device for regulating vane pump is disclosed in applicant's German Patent No. DE-OS No. 32 11 948, the U.S. counterpart being U.S. Pat. No. 4,536,133 issued Aug. 20, 1985 to applicant. In that patent a leaf spring is fastened to the front of the flow-regulating piston to control flow through choke bores 14,15. During the movement of the flow-regulating piston, the free end of the leaf spring effects reduced flow through the choke bore(s). Thus, a decreasing characterisic curve of the flow of oil is obtained through the entire speed range after a certain speed cutoff point (28 on FIG. 8 of U.S. Pat. No. 4,536,133) is reached. Consequently, a predetermined characteristic curve pattern can be effected.